This disclosure generally relates to the preparation of a semiconductor material substrate, especially a silicon wafer, which is suitable for use in the manufacture of electronic components. More particularly, the present disclosure relates to a process for the treatment of silicon wafers to form a high density non-uniform distribution of oxygen precipitate nuclei therein such that, upon being subjected to the heat treatment cycles of essentially any arbitrary electronic device manufacturing process, the wafers form oxygen precipitates in the bulk and a precipitate-free zone near the surface.
Single crystal silicon, which is the starting material for most processes for the fabrication of semiconductor electronic components, is commonly prepared with the so-called Czochralski (CZ) process wherein a single seed crystal is immersed into molten silicon and then grown by slow extraction. Molten silicon is contaminated with various impurities, among which is mainly oxygen, during the time it is contained in a quartz crucible. At the temperature of the silicon molten mass, oxygen comes into the crystal lattice until it reaches a concentration determined by the solubility of oxygen in silicon at the temperature of the molten mass and by the actual segregation coefficient of oxygen in the solidified silicon. Such concentrations are greater than the solubility of oxygen in solid silicon at temperatures typical for the processes used to fabricate electronic devices. As the crystal grows from the molten mass and cools, therefore, the solubility of oxygen in it decreases rapidly, whereby in the remaining slices or wafers, oxygen is present in supersaturated concentrations.
Thermal treatment cycles which are typically employed in electronic device manufacturing processes can cause the precipitation of oxygen in silicon wafers which are supersaturated in oxygen. Depending upon their location in the wafer and their relative size, the precipitates can be harmful or beneficial. Small oxygen clusters are electrically active thermal donors and can reduce resistivity regardless of location in the wafer. Large oxygen precipitates located in the active device region of the wafer can impair the operation of the device but when located in the bulk of the wafer, however, are capable of trapping undesired metal impurities that may come into contact with the wafer during, for example, device fabrication processes. This is commonly referred to as internal or intrinsic gettering (“IG”).
Rapid thermal process that reliably and reproducibly form a distribution of oxygen precipitate nuclei that comprise crystal lattice vacancies which, in turn, establish a template for oxygen precipitation in silicon wafers have been developed (see, e.g., the work of Falster et al. described in U.S. Pat. Nos. 5,994,761; 6,191,010; and, 6,180,220; each of which is incorporated herein by reference for all relevant and consistent purposes). The “ideal precipitating process” generally yields a non-uniform distribution of oxygen precipitate nuclei, with the concentration in the wafer bulk being higher than in a surface layer. Upon a subsequent, oxygen precipitation heat treatment, the high concentration of nuclei in the wafer bulk form oxygen precipitate nucleation centers which aid in the formation and growth of oxygen precipitates with the concentration of nuclei in the near-surface region being insufficient to do so. As a result, a denuded zone forms in the near-surface region and oxygen precipitates, sometimes referred to as bulk microdefects or simply BMDs, form in the wafer bulk.
Denuded zones depth may be controlled by controlling the cooling rate of the wafer from the anneal temperature to the temperature at which crystal lattice vacancies become essentially immobile for any commercially practical time period. Thin denuded zones may be desirable over relatively thick denuded zones, particularly in high resistivity wafers as, in general, oxygen removal efficiency decreases with increasing denuded zone depth because the distance over which the interstitial oxygen must travel in order to be removed from solution (either by precipitating at a BMD or by diffusing to the wafer surface) increases. As a result, once a denuded zone becomes too deep or thick, there is the potential that the elevated interstitial oxygen concentration in the center of this zone (interstitial oxygen near the surface and bulk of the wafer having sufficient time to diffuse to sites where they are consumed) will be sufficiently high, such that thermal donor formation will occur during a device manufacturing process, thus decreasing resistivity in the device layer of the wafer. This may be particularly problematic in wafers having deep denuded zones. Furthermore, the cooling rates required to achieve thin denuded zones are somewhat extreme and the thermal shock may create a risk of shattering the wafer.
A continuing need exists for new methods for producing ideal precipitating wafers, the methods allowing for less robust rapid thermal anneals to be performed thereby increasing processing throughput and/or methods that result in formation of thin denuded zones without increasing the cooling rate of the wafer.